Thermal imaging devices are frequently implemented with microbolometer infrared detectors provided, for example, in a focal plane array (FPA). Each microbolometer may be associated with a corresponding pixel of a captured image. Incident radiation received by each bolometer causes a resulting voltage to be captured during an image sampling period. The voltage is converted to a pixel value (e.g., pixel count) associated with the individual pixel.
Although microbolometers are effective infrared detectors, they typically require radiometric calibration to ensure that the captured pixel values properly correlate to temperatures of an imaged scene. For example, the performance of individual microbolometers and their associated circuitry may be evaluated at the factory during the time of manufacture to determine various correction terms to accurately correlate pixel counts with actual scene temperatures such that captured images are radiometrically calibrated.
Unfortunately, although microbolometers may be precisely calibrated at the factory, their performance can vary over time (e.g., ranging from seconds to months). If left uncorrected, these variations can lead to errors in radiometric accuracy.
In one corrective approach, local sensors are used to take temperature measurements of the FPA or other portions of the imaging device itself. These temperature measurements may be used by appropriate predictive processes to select various correction terms to compensate for changing microbolometer performance at different temperatures. However, such predictive processes may have limited accuracy and can require the implementation of multiple temperature sensors that may be difficult to implement.
In another corrective approach, a shutter providing a uniform black body of a known temperature may be intermittently introduced in front of the FPA to calibrate the microbolometers. Although this is generally effective, such an approach complicates the implementation of the imaging device with additional cost and development time, increased size and weight, and the possibility of shutter mechanical failure. Moreover, if additional components (e.g., optical elements such as lenses and/or protective windows) are provided between the imaged scene and the shutter, then such an approach may not account for radiometric variations attributable to such additional components.